two lasers with hector slam

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You can publish both to the scan topic. The sensor_msgs/Laserscan message that both sensors use contain the data that the hector_mapping needs, specifically the frame IDs of the sensors. I have not tried using two sensors with hector_mapping myself, but as long as you have you transform tree set up properly I think it should work.

Alternatively, you can use the relay node of the topic_tools package to subscribe to both scan0 and scan1 and republish the data to scan, although that would result in more overhead.